I. Field of the Invention
The present invention relates to diamond drag bits having cylindrical polycrystalline diamond faced inserts imbedded in the cutting face of a drag bit.
More particularly, the present invention relates to the optimization of the geometry of the cutting face of cutting elements, particularly of the type in which a diamond layer or other superhard material is adhered to a cemented carbide substrate to form a composite, and the composite is bonded to a support stud or cylinder. Alternately the support cylinder can be an integral part of the diamond substrate backing.
II. Description of the Prior Art
One type of cutting element used in rotary drilling operations in subterranean earth formations comprises an abrasive composite or compact mounted on a support cylinder or stud. The composite typically comprises a diamond layer adhered to a cemented carbide substrate, e.g., cemented tungsten carbide, containing a metal binder such as cobalt, and the substrate is brazed to the support cylinder or stud. Alternately, the support cemented tungsten carbide cylinder may be integrally formed as part of the polycrystalline diamond substrate backing. Mounting of these cutting elements in a drilling bit is achieved by press fitting, brazing or otherwise securing the stud or cylinder backing into pre-drilled holes in the drill bit head.
Fabrication of the composite is typically achieved by placing a cemented carbide cylinder into the container of a press. A mixture of diamond grains and a catalyst binder is placed atop the substrate and is compressed under ultra-high pressure and temperature conditions. In so doing, the metal binder migrates from the substrate and "sweeps" through the diamond grains to promote a sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form a diamond layer and also bonded to the substrate along a planar interface. Metal binder (e.g. cobalt) remains disposed within the pores defined between the diamond grains.
A composite formed in this manner may be subject to a number of shortcomings. For example, the coefficient of thermal expansion of the cemented tungsten carbide and diamond are somewhat close, but not exactly the same. Thus during the heating or cooling of the composite in the manufacturing process or during the work cycles the cutter undergoes in the drilling process creates significantly high cyclic tensile stresses at the boundary of the diamond layer and the tungsten carbide substrate. The magnitude of these stresses is a function of the disparity of the thermal expansion coefficients. These stresses are quite often of such magnitude to cause delamination of the diamond layer.
This limitation has been greatly minimized by adding a transition layer of mixed diamond particles and pre-sintered tungsten carbide between the full diamond layer and the carbide substrate, as taught by U.S. Pat. Nos. 4,525,178 and 4,604,106 assigned to the same assignee as the present invention and incorporated herein by reference.
Another shortcoming of state of the art diamond composite compact technology described above is the difficulty of producing a composite compact with any shape other than a flat planar diamond cutting layer that has low enough residual tensile stresses at the diamond/carbide interface that will permit its use as a drilling tool.
Using the technology of the above described U.S. patents, it is relatively simple to produce diamond composite compacts with concave, convex or other non flat cutting surfaces. This allows much greater freedom of design of drag type diamond compact drilling bits that are fitted with diamond cutters having significantly greater impact strengths and wear resistance. This technology is taught in U.S. Pat. No. 4,858,707. This patent is also assigned to the same assignee as the present invention and incorporated herein by reference.